Spectro-electrochemical studies on luminescent complexes

This thesis is devoted to the characterisation of redox-active metal-coordination complexes containing the M-bipy chromophore, where M = metal and bipy = 2,2'- bipyridine. Their electronic structural formulations have been closely defined by comparative examination of their absorption spectra in sequences of one-electron related oxidation states. We have found that such a series of related complexes offers a far greater chance of successful analysis of the absorption spectra than if the spectra are approached in isolation.The redox changes were generally achieved by controlled electrogeneration using an optically transparent thin layer electrode (O.T.T.L.E.) directly placed in the spectrophotometer beam, so that the absorption spectra of the unstable low-oxidation state complexes could be unambiguously recorded. It was found necessary to develop such 1spectroelectrochemical techniques' because of the extreme sensitivity to oxygen of the reduced complexes.In particular, we have shown that for reduced metal-bipyridyl complexes the spectroelectrochemical results can only be rationalised using a trapped-electron model; for example the tris-bipy complexes should be formulated as follows: [M(bipy)₃]ᶻ⁺ = [M(bipy°)₃]ᶻ⁺, [M(bipy)₃]⁽ᶻ⁻¹⁾⁺ = [M(bipy°)₂(bipy ⁻)]⁽ᶻ⁻¹⁾⁺, [M(bipy°)₃]⁽ᶻ⁻²⁾⁺ [M(bipy⁻)₃]⁽ᶻ⁻³⁾⁺. Thus we infer that in [M(bipy)₃]ᶻ⁺ and [M(bipy)₂L₂]²⁺ complexes the bipy ligands are noninteracting.We also discuss the metal-to-ligand charge transfer excited state of [Ru(bipy)₃]²⁺ in which we find the optically transferred electron to be exclusively located on one ligand; best formulated as *[Ru(III)(bipy°)₂ (bipy⁻)]²⁺ This complex is, therefore, an example of a symmetric D₃ ground state complex giving rise to a highly unsymmetric excited state.The detailed analysis of characteristic electrode potentials (for metal-based versus ligand-based reductions) and the spectroscopic recognition of bipy° and bipy⁻ chromophores have provided complementary and consistent electronic structural elucidations, from which molecular orbital energy diagrams can be usefully constructed, and a simple correlation between central metal charge and ligand-based reduction potentials can be identified